1. Field of the Disclosure
The present disclosure relates to a phase modulation apparatus which modulates a phase of light in a digital signal sequence.
2. Discussion of the Background Art
As a method of modulating light with a digital signal, Quadrature Phase Shift Keying (QPSK) and Differential Quadrature Phase Shift Keying (DQPSK) have been known (for example, see Patent Document 1).
In a long-haul transmission system, in order to suppress interference between codes and to realize the high sensitivity, a QPSK modulated or DQPSK modulated optical signal is sometimes further subjected to RZ (Return to Zero) intensity modulation. FIG. 1 is a view for explaining a phase modulation apparatus 300 employing an RZ-QPSK modulation method. The RZ-QPSK modulation is performed by the following procedure.
(1) Continuous light LD of a light source 10 is branched by a 1:2 coupler 11.
(2) Phase modulators (12-1, 12-2), based on the digital input signal (DATA1, DATA2) convert the continuous light (LD1, LD2)
into binary phase modulation optical signals (LN1, LN2), the phase of which is 0 or π.
(3) A 2:1 coupler 14 combines two phase modulation optical signals (LN1′, LN2) in a state that the phase of LN1′ is shifted from the phase of LN1 by π/2 phase by a phase shifter 13 and outputs a quadrature phase modulation signal QPSK.
(4) An intensity modulator 15 intensity-modulates the phase modulation signal QPSK with a clock signal CLK synchronized with the digital input signal and outputs an optical signal RZ-QPSK converted into an RZ signal.
The above procedure will be further described in detail in FIGS. 2 and 3. FIG. 2 is a view for explaining a process for generating the quadrature phase modulation optical signal from the binary phase modulation signal generated by phase modulators 1 and 2. In this example, an intensity modulator is used as a phase modulator. Namely, the amplitude of the digital input signal is set to twice a half-wave voltage of the intensity modulator, and, at the same time, a ‘0’/‘1’ level of the signal is set to a maximum transmission point of the intensity modulator, whereby digital ‘0’/‘1’ bits are converted into optical bits having a relative optical phase 0/π. Thus, the binary phase modulation optical signals LN1 and LN2 are generated. In this constitution, optical output is temporarily 0 at a phase transition point at which the optical phase is changed from 0 to π, or contrary to this. LN1 further becomes LN1′ in which the optical phase is shifted by π/2 by a π/2 phase shifter. LN1′ and LN2 are combined at a timing where the respective bits are overlapped, whereby QPSK phase modulation light in which the optical phase has four values of π/4, π/4, 5π/4, and 7π/4 is generated.
FIG. 3 shows eye diagrams of signals in a phase modulation apparatus 300. FIG. 3A shows the continuous light LD emitted from the light source 10. The light intensity of the continuous light LD does not change. FIG. 3B shows the binary phase modulation optical signal LN1 (LN2) output by the phase modulator 12-1 (12-2). Although the phase modulation optical signal LN1 maintains a constant light intensity in such a state that the optical phase is confirmed, the light is temporarily extinguished at the phase transition point as described above, and therefore, a notch N is generated in the eye diagram. FIG. 3C shows the quadrature phase modulation optical signal, QPSK, output from the 2:1 coupler 14. As described above, the two phase modulation optical signals (LN1′, LN2) are combined at a timing in which the respective bits are overlapped, whereby QPSK involves four phase states. Since the timing in which phase transition occurs is overlapped, the depth of the notch N is determined by two patterns depending on whether the phase transition occurs simultaneously in LN1′ and LN2 or whether the phase transition occurs in either one of LN1′ and LN2. FIG. 3D shows the RZ optical signal RZ-QPSK output from the intensity modulator 15. The RZ optical signal RZ-QPSK is obtained by applying RZ pulse modulation to a region where the optical phase is confirmed by a clock signal CLK synchronized with the digital input signal. As shown in the same drawing, this state is optimum when modulation is applied at a timing in which the peak of CLK overlaps an optimum phase (in between the notches). By virtue of conversion into the RZ signal, intersymbol interference between close signals is suppressed, and a high pulse peak value is obtained with respect to an average light intensity; therefore, a long-haul transmission system having high sensitivity and excellent nonlinear tolerance can be realized.
Patent Document 1: Japanese Patent Laid-Open Publication No. 2007-208472
An RZ-QPSK generating method will be described by such a procedure that two binary phase modulation lights are synthesized at a timing in which the respective bits of the binary phase modulation lights overlap to thereby generate quadrature phase modulation light, and RZ modulation is applied with a clock signal, using the center between the notches as an optimum phase. As seen from above, it is obvious in the RZ-QPSK generation, there is a problem that the timings of three kinds of signals including two data sequences (LN1′, LN2) and the clock CLK are aligned in parallel. The effect of a case where the timings between those signals are not aligned in parallel will be hereinafter described.
FIG. 4 shows the influence of deviation (skew between data and clocks) of the timing between the quadrature QPSK phase modulation light and CLK. When there is no skew, an output waveform is a repetitive pulse having constant time width as shown in FIG. 4A. However, when skew occurs, a notch and an RZ modulation clock interfere to remove a pulse, so that the output waveform becomes a distorted pulse as shown in FIG. 4B. Such distortion becomes lack of information to lead to significant deterioration of transmission characteristics.
FIG. 5 is a view for explaining the influence of the skew between data of the binary phase modulation light LN1′ and LN2 input to the 2:1 coupler 14. As described above, the quadrature QPSK phase modulation light includes a phase confirmation region R1 where four phase states are confirmed and a phase transition region R2 where the phase transition occurs and a notch is generated. When there is no skew, since the timings of phase transition between two signals are aligned in parallel equal, notches having different depths and the same width as shown in FIG. 5A are generated in the eye diagram. However, when the skew occurs, since multiplexing is performed while the timings of phase transition between two signals are deviated, the notch seen in the eye diagram is separated. Thus, as shown in FIG. 5B, the phase confirmation region is narrowed, and even if the clock phase is corresponded to the optimum timing, the interference with the phase transition region cannot be avoided, so that the transmission characteristics are significantly deteriorated.
Factors that the skew occurs between those three signals include variation in a delay time of individual parts (such as the modulator 12 and a driver) and wiring, delay variation depending on temperature and power supply variation, and variation in a long term. In the variation in the delay time of individual parts, the variation is individually adjusted by, for example, selection of parts having equal delay or adjusting by a phase shifter upon manufacturing, whereby it is possible to tentatively correspond to the variation in the delay time of individual parts. However, it is impossible to correspond to the delay variation due to temperature and power supply variation and variation in a long term only by initial adjustment, and such correction that operation is always performed at an optimum operating point is required.
In order to solve the above problem, this disclosure provides a phase modulation apparatus in which each component part is not required to have the same delay in the assembling and skew adjustment is easily performed even if temperature characteristics of parts and the change with time occur.